As the LSI technology tends toward higher integration and higher speed, further refinement of pattern rules is required. The light exposure currently employed as versatile technology is now approaching to the essential limit of resolution which is dictated by the wavelength of a light source. It is generally recognized that in light exposure using g-line (436 nm) or i-line (365 nm) as a light source, a pattern rule of about 0.5 .mu.m is the limit. LSIs fabricated by this technique have a degree of integration equivalent to 16 mega-bit DRAM at maximum. At present, LSIs fabricated in the laboratory have reached this stage. It is urgently required to develop a finer patterning technique.
One of approaches for making finer patterns is to use exposure light of a shorter wavelength in forming resist patterns. For the mass production process of 256 mega-bit (working size up to 0.25 .mu.m) DRAM (dynamic random access memory), active research works are now made to substitute a KrF excimer laser of lower wavelength (248 nm) for the i-line (365 nm) as the light source for exposure. However, for the manufacture of DRAM with a degree of integration of 1 G or greater requiring further finer processing technology (working size up to 0.2 .mu.m), a light source of further shorter wavelength is needed. In particular, photography using an ArF excimer laser (193 nm) is now under consideration.
In the case of lithography using light of a short wavelength of 220 nm or shorter as typified by an ArF excimer laser, in order to form fine patterns, photo-resists are required to have new properties which cannot be met by conventional materials. For this reason, since Ito et al. proposed a chemically amplified positive resist composition comprising a polyhydroxystyrene resin whose hydroxyl group is protected with a tert-butoxycarbonyloxy group (t-Boc group), known as PBOCST, and a photoacid generator in the form of an onium salt, a number of high sensitivity, high resolution resist compositions have been developed. Although these resist compositions have high sensitivity and high resolution, formation of a fine pattern having a high aspect ratio is deemed difficult when the mechanical strength of the resultant pattern is taken into account.
A number of resist compositions using polyhydroxystyrene as a base resin and having sensitivity to deep-UV, electron beams and x-rays are known in the art. These resist compositions rely on a single layer resist technique although a two-layer resist technique is advantageous in forming high aspect ratio patterns on stepped substrates. Because of such outstanding problems of substrate steps, light reflection from substrates, and difficult formation of high aspect ratio patterns, the known resist compositions are far from practical use.
It is known that the two-layer resist technique is advantageous in forming high aspect ratio patterns on stepped substrates. It is also known that in order to develop two-layer resist films with conventional alkali developers, silicone polymers having hydrophilic groups such as hydroxyl and carboxyl groups are required. Since silicone polymers having hydroxyl groups directly attached thereto, however, undergo crosslinking reaction in the presence of acid, it is difficult to apply such silanols to chemically amplified positive resist materials.
Recently, as the silicone based positive resist material capable of solving these problems, chemically amplified silicone based positive resist materials comprising polyhydroxybenzylsilsesquioxane, known as a stable alkali soluble silicone polymer, in which some phenolic hydroxyl groups are protected with t-Boc groups, and photoacid generators combined therewith were proposed (Japanese Patent Application Kokai (JP-A) No. 118651/1995 and SPIE, Vol. 1952 (1993), 377).
However, the polymers used in these silicone resist materials have aromatic rings, which cause substantial light absorption at a wavelength of 220 nm or shorter. Thus these prior art resins as such cannot be applied to photography using light of a short wavelength of 220 nm or shorter. Since the majority of exposure light is absorbed at the surface of resist, exposure light does not penetrate through the resist to the substrate, failing to form a fine resist pattern. (See Sasago et al., "ArF Excimer Laser Lithography (3)--Resist Rating--," the preprint of the 35th Applied Physics Society Related Union Meeting, 1P-K4 (1989)).
One exemplary phenyl group-free base resin for silicone based positive resist materials is disclosed in JP-A 323611/1993. Since all hydrophilic groups such as carboxyl and hydroxyl groups necessary to enable alkali development have been protected in this base polymer, many protective groups must be decomposed before exposed areas can be dissolved in the developer. Then the amount of photoacid generator added to this end is increased and the sensitivity is exacerbated. In addition, the decomposition of many protective groups has the high possibility of causing a change of film thickness and generating stresses or bubbles in the film. A resist material having high sensitivity and suited for fine patterning is not available.